Engineers typically design high-pressure oil field pumps in two sections; the (proximal) power end and the (distal) fluid end. The power end usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. Commonly used fluid ends usually comprise a pump housing having at least one suction valve, at least one discharge valve, and at least one bore for a plunger or piston, plus high-pressure seals, retainers, etc. FIG. 1 is a cross-sectional schematic view of a typical fluid end showing its connection to a power section by stay rods. A plurality of fluid sections similar to that illustrated in FIG. 1 may be combined in a fluid end, as suggested in the Triplex fluid end housing schematically illustrated in FIG. 2.
Valve terminology varies according to the industry (e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the moving element or valve body. In the application, however, the term “valve” includes not only a valve body but also one or more valve guides to control the motion of the valve body, a valve seat, and a valve spring and spring retainer that tend to hold the valve closed (i.e., with the valve body reversibly sealed against the valve seat). Valve bodies typically comprise guide means such as a crow-foot guide, a lower guide stem and/or a top guide stem for guiding the valve body as it moves between open and closed positions. Additionally, valve bodies typically include at least one seal retention groove for incorporating a peripheral element for sealing against a valve seat.
FIG. 3 schematically illustrates a cross-section of a web valve seat and a stem-guided valve body incorporating an elastomeric seal insert conventionally bonded within a seal retention groove. Conventional web-seat, stem-guided designs were proposed in the past to withstand the high pressures and repetitive impact loading typical of oil field service. Elastomeric seal tearing or cracking, as schematically illustrated in FIG. 3, extrusion of elastomer into the extrusion gap, and excessive wear of lower and/or top valve stem guides are among the common failure modes of these valves. Additional background, particularly on extrusion-related elastomer stress, can be found in U.S. Pat. No. 6,955,181 B1, incorporated herein by reference.
In valves with conventionally bonded elastomeric seal inserts analogous to that schematically illustrated in FIG. 3, elastomeric seal tearing or cracking typically occurs during valve closure. Such seal damage occurs in part because of high residual elastomer stress that develops due to shrinkage of the elastomer as it cures. Since the seal insert is conventionally bonded to the metal walls of the seal retention groove, elastomer near the bond cannot move appreciably when a cast-in-place seal insert shrinks during curing. In contrast, elastomer more distant from the bond is more free to move as it shrinks
Thus, a high residual level of stress is established between areas of seal elastomer with different degrees of movement. And this stress is exacerbated when the seal insert strikes the valve seat. By design, this insert-seat contact occurs slightly before the impact area of the valve body strikes the valve seat. As the valve body then continues to advance toward the valve seat, the portion of the peripheral seal in contact with the valve seat is dragged down the face of the valve seat, thus simultaneously increasing seal elastomer stress. This dragging motion is impeded by friction between the peripheral seal and the valve seat, such friction often being increased by particulate matter trapped between the peripheral seal and the valve seat during valve closure. The combination of frictional drag forces, extrusion stress and seal abrasion due to trapped particulates so increases overall seal elastomer stress that it predisposes the peripheral seal to tearing or cracking. At the same time, valve seat wear is also increased.
Excessive valve guide stem wear is another possible failure mode of valves having top-stem-guided valve bodies such as discharge valve body 701 shown in FIGS. 4A and 4B. Such valves are schematically illustrated in copending patent application Ser. No. 11/125,282, which is incorporated herein by reference. FIG. 5 is a partial cross-section schematically illustrating discharge valve body 701 in its closed position (i.e., with elastomeric seal 703 held in symmetrical contact with valve seat 705 by discharge valve spring 707). Note that top guide stem 709 of discharge valve body 701 is aligned in close sliding contact with top valve stem guide 711.
FIG. 6 schematically illustrates how misalignment of top guide stem 709 is possible with excessive wear of top valve stem guide 711. Such excessive wear can occur because discharge valve body 701, including top guide stem 709, is typically made of steel that has been carburized to a hardness of about 60 Rockwell C. In contrast, the wall of top valve stem guide 711, which is shown in FIG. 6 as being formed within discharge bore plug 713, is typically made of mild alloy steel with a hardness of about 30 Rockwell C. Thus the softer wall of stem guide 711 is worn away by sliding contact with the harder guide stem 709. This wear is accelerated by side loads on valve body 701 that result when fluid flowing past the valve body changes its direction of flow into the discharge manifold. Analogous side loads would be present on a suction valve when fluid flowing past the valve body changes its direction of flow into the plunger cavity.
Eventually, top valve stem guide 711 can be worn sufficiently to allow discharge valve leakage due to significant asymmetric contact of elastomeric seal 703 with valve seat 705 as schematically illustrated in FIG. 7. This problem of stem guide wear is typically addressed in practice through use of a replaceable bushing 715 having a modified top valve stem guide 711′ (see the schematic illustration in FIG. 8). Bushing 715 is commonly made of a plastic such as urethane, or a wear and corrosion-resistant metal such as bronze. Such bushings require periodic checking and replacement, but these steps may be overlooked by pump mechanics until a valve fails prematurely. Hence, it has been proposed to replace the carburized steel top guide stem 709, as well as peripheral seal 703, with a guide stem and seal comprising one or more relatively resilient and substantially non-metallic materials having a relatively low specific gravity. See, e.g., U.S. Pat. No. 4,860,995 (hereinafter the '995 patent) wherein a plastic or plastic-like insert is described as being distorted sufficiently to engage and then be mechanically locked, or alternatively bonded, (or both mechanically locked and bonded) to the body portion of a valve element (see col 3, lines 54-68 and col 7, lines 39-57).
If preformed seal inserts are to be distorted and mechanically locked to a valve body as in the '995 patent, the valve body requires finish machining to closely match the dimensions of the seals. Manufacturers recognized that this finish machining could be reduced or eliminated if elastomeric seals were cast and cured in place on the valve body (hereinafter “cast-in-place”). But savings in machining costs were often offset in practice by added costs associated with adhesive bonding of the cast-in-place seal inserts to a valve element in an attempt to increase overall valve body integrity (see the '995 patent, col 7, lines 47-50). The added costs of adhesive bonding, including removal of all oil and contaminants, application of a bonding adhesive, and storage of the valve bodies in a low-humidity, dust-free environment while awaiting the casting, bonding, and curing of the seal insert increased the cost of such valves to the point that they were not competitive on price. Further, as shown in FIG. 3, the elastomer of such cast-in-place seal inserts was subject to cracking or tearing where it was adhesively bonded to a peripheral valve body groove.